The present invention relates generally to a modulation scheme for driving a load and, more particularly, systems and methods to implement a modulation scheme for a switching amplifier that can reduce filtering requirements and crossover distortion.
There are two common approaches utilized to drive a load with a desired voltage or current. These are using a switching amplifier or a linear amplifier to drive the load. Various types of loads can be driven by these approaches, including, for example, motors or actuators, thermoelectric coolers and/or heaters, and audio speakers, to name a few.
In the switching amplifier approach, output transistors are switched, or pulse-width modulated, at a frequency usually much higher than the frequency of interest in an application. The result is that the transistors are on only when they need to supply current. The pulses vary in duration to change the amount of voltage or current being applied. The longer the pulse, the more energy applied to the load and the shorter the pulse, the less energy applied to the load. Thus, varying the length of the pulse can control power provided to the load and its associated operation.
By way of example, a typical PWM approach employs a pair of high-side and a pair of low side transistors in which one or more transistors at each of the high side and low side are pulse-width modulated to drive the load in a desired manner. The switching of the transistors tends to introduce undesirable noise to the load and surrounding circuitry. Accordingly, filters are needed to reduce switching noise. Further, because both sides implement switching in the conventional switching amplifier approach, a filter is needed at each of the sides of the load to reduce switching noise. Such filters can be significantly large, often occupying relatively large amounts of real estate on a printed circuit board (PCB).
The primary advantage of a switching amplifier is high efficiency, which allows for lower heat dissipation. A primary disadvantage of a switching amplifier is that it typically requires a bulky and/or costly output filters to block the switching waveform from reaching the load.
The switching amplifier is in contrast to a linear amplifier or regulator, which acts like variable resistor between the power supply voltage and the load to provide a desired voltage or current. The linear amplifier controls or drives the load with a controlled current or voltage, such as to control operation of the associated load. The linear driver, for example, operates one or more transistors in a linear region so as to achieve a desired signal to the load, such as a variable DC output. However, such operation tends to be quite inefficient because substantial power is dissipated when operating in this manner.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention relates generally to a modulation scheme that can be utilized to drive an associated load coupled between a pair of output nodes. In this scheme, controlled output signals are provided to each of the output nodes. The output signal provided at one output is a switching signal and the other output is non-switching. Because switching is implemented at only one side of the load, a single filter may be employed at the output to the load. As a result of using only a single filter, the area needed to implement a corresponding circuit can be reduced.
By way of example, such a scheme can be implemented to control a plurality of switching devices, such as arranged in a bridge configuration, to selectively energize a load coupled between a pair of output nodes. The switching devices can be coupled between a single power supply and ground. Respective switching devices associated with one of the output nodes are controlled in a switching mode (e.g., by pulsing or pulse-width modulating such switching devices) to provide a switching signal at that node regardless of current direction. A filter can be coupled between the load and this output node (e.g., the juncture between the respective switching devices). The switching devices associated with the other output node are controlled in a non-switching mode (e.g., by operating the switching devices in a steady state condition, such as on or off), which operation controls the direction of current flow through the load and provides a non-switching output signal at this node.
According to another aspect of the present invention, crossover distortion typically associated with a zero crossing condition can be reduced by providing a generally linear low voltage signal at one or more of the output nodes, such as by operating associated switching devices in linear modes. The switching devices employed in the linear mode can be part of a separate switching system or include the same switching devices that implementing the switching modulation scheme described above.